Many jurisdictions within the United States and foreign countries have established regulations for the gross weight of a highway vehicle, such as a semi-trailer. The gross weights permitted are typically related to the vehicle size, and particularly to the vehicle wheelbase.
In particular, the gross weight permitted is often related to the number of axles on the vehicle and the wheelbase defined as the distance between selected axles. In the common semi-trailer type configuration including a power unit and a trailer, the wheelbase can be measured a number of ways. The wheelbase can be measured between the leading and trailing axles of the trailer, between the leading drive axle of the power unit and the trailing axle of the trailer or between the steering axle of the power unit and the trailing axle of the trailer.
A careful consideration of the axles used to determine the allowable gross vehicle weight is necessary to pick the optimal wheelbase measurement to permit the maximum payload to be carried in the vehicle. In optimizing this measurement, the total weight and weight distribution of the power unit and trailer must be considered.
Because the power unit and trailer of the vehicle are commonly designed and constructed to carry a much greater payload than that permitted by law, it is desired to design a vehicle for the maximum legal wheelbase, permitting a large payload to be carried, while minimizing the problems associated with lengthening the length of the vehicle, such as stability and turning radius.
One commonly used vehicle is the "end dump"trailer, which is a dump truck designed for semitrailer operation. The end dump trailer will typically include a trailer body for containing the payload, a rear tandem wheel unit which pivotally supports the rear of the trailer body, and structure for pivoting the trailer body about the rear tandem wheel unit to tilt the trailer body for dumping. The end dump trailer can be a "frameless" type, which has no non-pivoting frame supporting the trailer body, or a "frame" trailer which incorporates a rigid non-pivoting frame to support the pivoting structure. The trailer is designed to permit the trailer body to be tilted to an angle of approximately 50.degree. from the horizontal for dumping operations. The rear tandem wheel unit supports the rear of the trailer. The drive axles of the power unit will support the front of the trailer. In operation, a body length of about 32 feet has been found to be a desirable maximum figure. Trailer body lengths of 35 and 37 feet are believed in the industry to be nonstable in the dumping position. Therefore, a need exists to design an end dump trailer which includes a trailer body length that is acceptable in the industry while maximizing the permitted gross vehicle weight and payload under the law.
In addition to the "end dump" trailer, a similar need exists with regard to the "roll off" trailer. This trailer operates in a manner very similar to the frameless trailer noted previously. However with the roll off trailer, a trailer body or container is slidable on or off the trailer in the tilted position.